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Female lizards produce two fewer offspring in average when they are forced into sex by outnumbering males.

Jean-François Le Galliard/University of Oslo

Ganging Up on the Girls

It seems that 9-year-old boys aren't the only male creatures who will join together to torment their female counterparts. When male lizards largely outnumber females, they direct their aggressiveness toward mating partners, population biologists report. Such belligerence, they say, could put lizard populations at risk of extinction.

Ecologists and evolutionary biologists have usually assumed that the ratio of males to females is stable in most animal populations. That's partly because in many species males compete for mating partners, and if there are too many males, the extra will die off or wander away in search of a mate. On the other hand, females are more sensitive to food shortages. These two factors are thought to help keep sex ratios essentially constant. But testing this theory has proven difficult, says Jean-François Le Galliard, an evolutionary population biologist at the University of Oslo in Norway.

Le Galliard and colleagues from the CNRS in Paris tackled the challenge by first monitoring the behaviour of common lizards, Lacerta vivipara, in outdoor enclosures for a year. Lizards were separated into two populations, each with about 70 members. In one population the adults were three-quarter males, and in the other they were three-quarter females. Lizards were allowed to emigrate to another population of the same bias in sex ratio. The mortality and emigration rates of male lizards were unaffected by sex ratio imbalances, the team reports online this week in Proceedings of the National Academy of Sciences. But females were 2 to 3 times more likely to die or be wounded by males when their environment was male-dominated than when it was female-dominated. The team concluded that rather than fighting off male competitors, the too-numerous male lizards forced the females into mating.

Although this strategy offered an immediate reward for males, it didn't pay off for the population as a whole: The fraction of females laying eggs did not increase in male-dominated versus female-dominated populations, and in male-dominated populations egg-laying females produced two fewer offspring on average. As a result, male-dominated populations shrank by up to 50% over the year and saw their sex ratio skew further. A computer model suggest that the effect could drive a population into extinction five times faster than might otherwise happen.

The results "show that an increase in the fraction of adults who are male can actually grow over time," says Shripad Tuljapurkar, a population biologist at Stanford University in California. They also suggest another factor to take into consideration in wildlife management measures, especially for endangered species, Tuljapurkar says: "We need to monitor trends in adult sex ratios and worry about their correlation with population trends."

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